Abstract
This study provides an experimental investigation on the effect of microcracks on the tensile properties of 3D woven composites. A four-step experimental procedure using the combination of micro-XCT, acoustic emission (AE) and digital image correlation (DIC) is here proposed. Typical tensile damage behaviors were characterized by the stress–strain curves, AE signal analysis and DIC full field strain measurement. Due to a typical four stages stress–strain behavior, phenomena of stiffness degradation and stiffness hardening were successively found during the tensile process. Samples with various damage levels were produced by the in situ AE monitoring. Their 3D microcrack morphologies show the crack initiation, propagation process and the damage modes. Detectable damages initiated during the stress range from 65.98% to 72.93% σs. The cracks volume fraction (CVF) shows a positive correlation relationship with the corresponding tensile load. Moreover, the CVF was used to characterize the degree of damage. The samples with various phased damages were tested again in the fourth step to obtain their residual modulus and residual strength. Detected microcracks have little influence on the residual strength, while the residual modulus witnesses a regular decrease along with the damage increase. The effect of microcracks on the tensile properties is characterized by the relationships between the gradually increased damages and the corresponding residual properties which provide a foundation for damage evaluation of 3D woven structures in service.
Highlights
Three-dimensional woven composites were developed in the 1960s to overcome the shortcomings of traditional laminates and have been paid more and more attention to, due to the outstanding mechanical properties and to their higher production efficiency and net-shape manufacturing with complex geometry designs [1,2,3]
Acoustic emission (AE), digital image correlations (DIC) and X-ray based computer tomography (CT) are the three methods usually adopted in open literatures to characterize the mechanical behaviors and damage mechanisms
The typical tensile damage behavior of 3D woven composites is investigated with a four-step experimental method showing the initiation and propagation of microcracks and the effect of microcracks on the tensile properties
Summary
Three-dimensional woven composites were developed in the 1960s to overcome the shortcomings of traditional laminates and have been paid more and more attention to, due to the outstanding mechanical properties and to their higher production efficiency and net-shape manufacturing with complex geometry designs [1,2,3]. DIC was chosen by some researchers to map the surface strain field for a better understanding of the quasi-static mechanical properties and the deforming process of 3D woven composites, and their fatigue performances and damage mechanisms [28,29,30]. DIC only allows us to do damage evaluation from the sample surfaces and we cannot obtain any information that can characterize the inner damages Another NDT technique, X-ray based computer tomography, makes it possible to qualitatively and quantitively analyze the inner damage behaviors through visualization of the tomography photograph and the damage 3D reconstruction [31,32,33,34]. The typical tensile damage behavior of 3D woven composites is investigated with a four-step experimental method showing the initiation and propagation of microcracks and the effect of microcracks on the tensile properties.
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